Hand-held edge tools such as chisels and plane irons are usually supplied by the manufacturer with a flat bevel face lying at 25.degree. to the plane defined by the opposite major face of the tool, the edge of the tool is then honed at 30.degree. on a finer stone. Although large scale automatic machinery for grinding the bevel face and honing the edge of a hand tool are known, equipment available for the owner of a set of hand tools to sharpen these is primitive in the extreme. Indeed, the conventional technique for sharpening hand-held edge tools comprises stroking or rubbing the edge on a flat stationary stone to form the honed edge. Repeated sharpening operations result in the honed edge growing very much larger as the material is worn away, so that the bevel face becomes considerably reduced. Re-grinding the bevel face is a time consuming and tedious operation, and attempts to perform this by hand are almost inevitably unsatisfactory due to the inability of the operator to maintain the angle of the tool precisely constant during the whole of the stroke required to provide relative motion between the stone and the tool. Unless the operator is highly skilled even the honing operation performed on the very edge of the tool is insufficiently accurate when performed by hand to provide a satisfactory sharpened edge, and vary often the honed tip is rounded due to the tendency of the operator to rock the tool slightly during its forward stroke when in contact with the stone.
Various attempts have been made to produce apparatus suitable for an individual tool owner to sharpen edge tools by mechanical means. One such tool is described in British Patent No 1293729, which shows the provision of a pair of rotary grindstones one at each end of a motor shaft in a configuration similar to a conventional bench grinder, in front of which is positioned a tool mount carried on a transverse guide bar extending parallel to the axis of rotation of the grindstones. An edge tool can then be fitted on the mount and is displaceable across the cylindrical surface of one or other of the rotary grindstones held rigidly in a predetermined orientation during grinding. This known tool has the disadvantage that, because it is passed over the cylindrical surface of the grindstone the face ground on the bevel edge of the tool is concavely curved to form a so-called hollow ground face, which weakens the edge of the tool and allows it to become blunt more rapidly than an edge defined by meeting flat surfaces. There is also the possibility that a variation in pressure applied by the user as the edge tool is traversed across the grindstone will result in differing amounts of material being removed from the edge of the tool at different transverse positions so that a straight true edge is not always achieved, especially for wider tools such as plane irons. In an attempt to overcome this problem the tool holder described in European Patent application 225806 incorporates a stop member for limiting the rotation of a tool holder about a tool guide formed as a cylindrical bar, but this tool too provides a bevel face which is hollow ground and, furthermore, involves considerable complexity in the tool mount.
Flat bevel faces can be ground on the edge of a tool by the machine described in UK Patent No. 1093220. This, however, is a commercial machine for grinding a bevel face on a plurality of tufting knives used for cutting the pile in a tufting machine. A cup-shape grindstone is driven to rotate closely adjacent a tool holder in the form of a frusto-conical plate having a plurality of grooves in its conical surface, into which grooves several individual tufting knives may be fitted, to be clamped by a surrounding band. Up to 16 such grooves are provided and packs of up to 23 knives may be carried in each groove allowing up to 368 knives to be sharpened simultaneously upon rotation of the tool holder. The depth of cut of the grindstone which determines the amount of material removed from the edge of each tool as it is traversed over the flat annular face of the cup-shape stone is determined by adjusting the position of the holder along its axis of rotation. This machine is complex and unsuitable for use by an individual wishing to sharpen a single hand tool.